Turn milling of casings takes advantage of the component only needing to rotate very slowly while the milling cutter can work at maximum cutting speed. The turn milling process has enabled the efficient manufacturing of many components which would otherwise have been difficult to make and needed a number of different machining operations, in different set-ups, in different machines. The process excels when components have unsymmetrical forms and with surfaces less than 360 degrees round the part surface because of flats, cavities and obstacles.
Although this is the main advantage, other turn-mill advantages include improving chip control, lowering cutting forces, higher capability for demanding workpiece materials and coping better with interrupted cuts. Both machinery, programming and cutting tools have undergone development to make turn milling a very viable and advantageous process.
Many of today’s standard CoroMill cutters – such as the CoroMill 345 – are ideal for turn milling and can be equipped with dedicated wiper inserts. There are tools suitable for optimizing the different configurations that keep arising as suitable for turn milling.
Sandvik Coromant has developed tools and methods to take proper control of the process parameters and thereby lift turn milling to broader use, to new levels of efficiency and to achieve better quality results. Process development and application have provided a full understanding of how the various parameters affect performance and results, providing a key to this interesting potential in manufacturing.
Turn milling rarely requires dedicated tools for roughing operations. But for optimizing finishing operations, dedicated, standard wiper indexable inserts for the CoroMill 345 should be used. A straight wiper cutting edge, as opposed to the conventional cambered one for general milling, is needed to smooth the slight surface form deviations. Round insert cutters like the CoroMill 200/300 offer the best productivity when surface flatness is not paramount, such as when roughing casings.
Application technology is needed because factors such as the positioning of the milling cutter in relation to the workpiece is critical as regards radial form accuracy, pressure on workpiece and security as well as productivity of the operation.
Optimized turn milling
The positive effects of optimized turn milling is illustrated by the method applied to machining the grooves for vane rings in turbine casing-halves at one of the world’s largest gas turbine makers. Questioning the conventional use of disc-type, side-and-face milling cutter, individual grooves were machined through face turn-milling instead. Each of the milling cutters used can be adjusted to suit each application and are simpler to use than the disc-type cutters as well as being more flexible in the machine. The method also produces grooves to better precision.
The application of turn milling was part of a productivity improvement programme with Sandvik Coromant as the process optimizer in the company’s rotor and housing machine shops. The alternative, fine-tuned process could be applied that improved productivity by 40%.